Method and equipment for multirate coding and detection in a multiple access mobile communication system

ABSTRACT

In a communication system in which data and user information are transmitted on a radio path between a transmitting end and a receiving end, multirate coding and detection are carried out by: allocating different signal waveforms to different transfer rates, selecting, at the transmitting end, the data transfer rate to be used on the radio path for transferring user information, selecting, at the transmitting end, the signal waveform to be used on the radio path according to the selected transfer rate, transmitting the user information with the selected transfer rate and the selected signal waveform to the receiving end, the signal waveform indicating the selected data transfer rate, detecting, at the receiving end, the signal waveform used on the radio path, selecting, at the receiving end, a transfer rate corresponding with the detected signal waveform, and adapting reception to the selected transfer rate.

This application is the national phase of international applicationPCT/FI96/00255, filed May 7, 1996 which designated the U.S.

FIELD OF THE INVENTION

The present invention relates to multirate coding and detection in amultiple access mobile communication system in which a transmitterselects the data transfer rate, and forwards information on the selecteddata transfer rate to a receiver which adapts reception to the selectedrate.

BACKGROUND OF THE INVENTION

In present-day digital mobile communication systems, user information,i.e. speech and data, are transferred over the radio interface indigital form. In some cases, it is possible to select out of manytransfer rates the one which is best suited to a speech coding rate usedand to data transfer requirements of the user. The transfer rate istypically chosen at the beginning of a call and maintained unchanged forthe duration of the call. In mobile communication systems to come, moreflexibility is required at the radio interface for embodying variouskinds of services. One of the consequences of this requirement forflexibility is a rapidly varying transfer rate during a call. In adigital mobile communication system, for example, in which userinformation is packed into transmission frames (10 ms of duration, forexample), each frame may have a transfer rate independent of theprevious or the subsequent transfer frame. However, a problem emerges inhow to transfer information on the current transfer rate as quickly aspossibly from the transmitter to the receiver in order for the receiverto be able to adapt its operation to the transfer rate being used. Inaddition, in interference limited mobile communication systems such asspread spectrum radio systems, it is advantageous to employ a fastclosed loop power control. By means of power control, a base stationsseeks to adjust the transmit power of mobile stations so that all thesignals transmitted by the mobile stations are received by the basestation at the same nominal power level. In other words, by means ofpower control, the aim is to average the energy of a symbol transmittedover the radio path, and therefore the transmit power is alsoproportional to rate. Due to this, when employing closed loop powercontrol, the receiver needs to know the current transfer rate withoutdelay.

A known solution is to employ on the radio path a separate signallingchannel through which information on the transfer rate is conveyed fromthe transmitter to the receiver. The closed loop power control can becarried out with such a signalling channel. Separate signalling,however, causes delay in the practical implementation. The messageindicating transfer rate is protected against interference andinterleaved in the transmitter. The most efficient interleaving lastsfor the length of the transfer frame. Prior to knowing the transfer rateand completing the user signal processing, deinterleving and errorcorrection are carried out in the receiver. As a consequence thereceived user signal has to be buffered until these reception operationsare completed. A further consequence is that activating the closed looppower control is delayed. If the receiver of a spread spectrum systemutilizes interference cancellation (IC) or multiuser detection (MUD),the delay caused by detecting the transfer rate employed also concernsthe interfering signals (other users). In an asynchronous spreadspectrum mobile communication system, the propagation delay, with theabove assumptions, will be twice the interleaving depth+signalprocessing delay.

DISCLOSURE OF THE INVENTION

It is an object of the present invention to provide a system and anapparatus by means of which a receiver of a mobile communication systemis able to detect, with a reasonable delay and an economical receiverconstruction, the transfer rate employed by a transmitter at any onetime.

This is achieved with a method comprising the steps of selecting, at thetransmitting end, the data transfer rate to be used on the radio pathfor transferring user information; forwarding information on the currenttransfer rate to the receiving end; adapting reception to the transferrate employed. The method of the invention is characterized byallocating different signal waveforms to different transfer rates;selecting, at the transmitting end, the signal waveform to be used onthe radio path according to the selected transfer rate; detecting, atthe receiving end, the signal waveform used on the radio path;selecting, at the receiving end, a transfer rate corresponding with thedetected signal waveform.

The invention also relates to an equipment for multirate coding in atransmitter of a multiple access mobile communication system, whichtransmitter sends user information to the radio path with a varyingtransfer rate. The equipment of the invention is characterized in thatthe transmitter is arranged to allocate different signal waveforms todifferent transfer rates, and the equipment is arranged to convey a datatransfer rate information to the receiving end by using a signalwaveform allocated to the transfer rate on the radio path.

In addition, the invention relates to an equipment for detecting avarying transfer rate in a receiver of a multiple access mobilecommunication system, the receiver receiving user information from theradio path with a varying transfer rate. The equipment of the inventionis characterized in that the receiver is arranged to allocate differentsignal waveforms to different transfer rates, and that the equipmentcomprises a detector which detects the transfer rate used at thetransmitting end on the basis of the received signal waveform.

According to the invention the varying transfer rate is coded at thetransmitting end by using a signal waveform which is selected accordingto the transfer rate employed at any one time. At the reception end, thewaveform of the received signal is recognized, and, consequently, thetransfer rate employed by the transmitter will be recognized. Forexample, in a mobile communication system utilizing frequency hopping(FH) or time hopping (TH), the hopping pattern may be selected in thetransmitter according to the transfer rate. In a multi-carrier HFDMAsystem the carrier waves used, or combinations thereof, may be selectedaccording to the transfer rate. In a CDMA system, the spreading code maybe selected according to the transfer rate. In addition, these methodscan be combined.

In other words, methods similar to those being used to distinguishdifferent users from one another are employed for the recognition of thetransfer rates of different users. It is characteristic for theaforementioned multiple access methods that there is a higher number ofproper spreading waveforms available than the number of simultaneoususers allowed by the interference limitations of the mobilecommunication system. Due to the above, the mobile communication systemhas extra “channels” available by means of which the “multiple access”can be extended to cover transfer rate as well. However, it must benoted that the multiple access methods and waveforms employed forrecognizing the users in a mobile communication system from one anothermay be based on a different method than the transfer rate codingaccording to the present invention in the same system.

One of such methods differing from the multiple access methods is toshape the user information with Rademacher waveforms or similarwaveforms that have adequate cross correlation characteristics. This isan economical method as far as the implementation of a receiver,particularly in a CDMA mobile communication system, is concerned.

As the different transfer rates are identified according to theinvention by means of different signal waveforms, the signal waveformsand, consequently, the transfer rate can be detected in the receiverafter reception of only a few symbols (speech or data). The detection ofa user signal can thus be started in the receiver after a relativelyshort delay without having to wait, e.g., for the end of the frame anddeinterleaving. As a result, it is possible to activate closed looppower control and multiple access interference cancellation of, forexample, spread spectrum systems from almost the very beginning of aframe. At first, in the beginning of the frame the most probabletransfer rate may be estimated by following the maximum principle, forexample, until it is possible to make a reliable final decision on thetransfer rate.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, the invention will be described in closer detail bymeans of the preferred embodiments with reference to the accompanyingdrawings, in which

FIG. 1 is a block diagram of a CDMA transmitter in which the spreadingcode is selected on the basis of the transfer rate,

FIG. 2 is a block diagram of a CDMA receiver in which the transfer ratecoded by the transmitter of FIG. 1 is detected on the basis of thespreading code,

FIG. 3 is a block diagram of a CDMA transmitter in which a transfer rateis coded by shaping a user data code with Rademacher waveforms,

FIG. 4 is a block diagram of a CDMA receiver which detects the transferrate coded in FIG. 3 on the basis of the Rademacher waveform, and

FIGS. 5A, 5B, 5C and 5D show a signal chart illustrating the operationof the transmitter of FIG. 3 and the receiver of FIG. 4.

PREFERRED EMBODIMENTS OF THE INVENTION

As noted in the above, the basic idea of the invention is to code thevarying transfer rate of user information (speech or data) in thetransmitter by using a spreading waveform which is selected according tothe transfer rate being employed at any one time. In the receiver, it isdetected at which spreading waveform the signal is present, and thetransfer rate used will be identified accordingly. Multiple accessmethods enable a simultaneous access for a multitude of users to thefrequency spectrum with minimum disturbance to one another. In afrequency division multiple access method (FDMA), each user has adedicated frequency channel, which is a relatively narrow frequency bandon which the transmit power of the user's signals is concentrated. In atime division multiple access method (TDMA), the channel consists of atime slot within a sequence of several time slots forming a frame. Theenergy of a user's signal is limited to one of these time slots. Somemobile communication systems employ a combination of FDMA and TDMAmethods.

FDMA, TDMA or FDMA/TDMA mobile communication systems may employfrequency hopping (FH) or time hopping (TH) to spread a user's signal infrequency or time domain. Frequency hopping utilizes a wider spectrum bychanging the carrier frequency (frequency channel) of the transmitsignal sequentially. Correspondingly, the time hopping method changesthe time slot (channel) of the transmit signal sequentially. Thischannel changing is referred to as “hopping”, and the channels and theirsequence within one hopping cycle as a hopping pattern.

Frequency or time hopping can be employed in the transmitter to code thetransfer rate according to the invention. In this embodiment differenttime or frequency hopping patterns are allocated to different transferrates, which can be selected during a call. Mapping (determinedco-dependence) between the hopping patterns and transfer rates is knownby both the transmitter and the receiver. The hopping pattern used atany one time is selected in the transmitter according to the transferrate. In the receiver, it is detected at which hopping pattern thesignal is present, and on the basis of the detected hopping pattern arespective transfer rate is identified. Following this, the receiveduser signal is detected and further processed by using said identifiedtransfer rate.

In a multi-carrier system (orthogonal FDMA), carrier waves or theircombinations used by the transmitter at any one time are selectedaccording to the transfer rate. Mapping between different transfer ratesand carriers, or their combinations, is known by both the transmitterand the receiver. The receiver detects at which carrier waves or theircombinations the signal is present and on the basis of this identifiesthe transfer rate. Following this, the receiver detects and processesthe user signal further at this transfer rate.

In a code division multiple access (CDMA) system, each user is providedwith a dedicated pseudo-random binary sequence, which is referred to asa spreading sequence. The user signal and carrier are modulated by thespreading sequence, resulting in a spread spectrum of the modulatedwaveform. This means that a plurality of CDMA signals can share the samefrequency spectrum. These signals are identified in the receiver byusing a correlator which combines the energy of a specific binarysequence and reproduces its original spectrum. The invention isapplicable to coding a transfer rate in a CDMA mobile communicationsystem by selecting the spreading code in the transmitter according tothe transfer rate.

The methods disclosed above may also be combined. In other words, thedifferent transfer rates of individual users can be distinguished fromone another by means of methods similar to those being used todistinguish different users from one another. The multiple accessmethods described above are characterized in that there is a highernumber of proper spreading waveforms available than the interferencelimitations of the system allow there to be simultaneous users. By meansof these “extra” spreading waveforms, the “multiple access” can beextended to cover different transfer rates of one subscriber also.However, it must be noted that this does not exclude the alternativethat the actual multiple access is provided in the mobile communicationsystem with a different method than the transfer rate coding accordingto the present invention. An example of such an alternative will bedescribed below in connection with FIGS. 3, 4 and 5.

In the following, the preferred embodiments of the present inventionwill be described in connection with a CDMA system, to which theinvention can especially well be applied. The CDMA principle will onlybe described to the extent necessary for illustration of the invention.For a more detailed description of CDMA, the document “An overview ofthe application of code division multiple access (CDMA) to digitalcellular systems and personal cellular networks”, QUALCOMM Incorporated,May 21, 1992, USA is referred to.

FIG. 1 is block diagram illustrating the principle of a CDMA transmitterin which a spreading code is selected according to the transfer ratebeing used at any one time. In accordance with the normal principle of aCDMA transmitter, the user signal 1, i.e. speech or data, obtained froma data source 1 is mixed with a spreading code from a spreading codegenerator 5, in a mixer 2. The spreading code spreads the user signal,producing a spread spectrum signal which is modulated in a transmitter 3to a carrier frequency F1, generated by an oscillator 6, and which istransmitted to the radio path via an antenna 7. The user signal transferrate of the data source 1 may vary rapidly during a call, obtaining twoor more different values. According to the invention, a dedicatedspreading code is allocated to each of the different transfer rates.Information on the mapping between the allowed transfer rates and thecorresponding spreading codes is stored in a control unit 4. Duringtransmission, the control unit 4 selects, according to the transfer rateemployed by the data source, a corresponding spreading code which thespreading code generator 5 generates for the mixer 2. Thus, thetransmitter of FIG. 1 transmits to the radio path a spread spectrumsignal whose spreading waveform depends on the transfer rate.

FIG. 2 shows a block diagram illustrating the principle of a CDMAreceiver in which the transfer rate can be detected according to thespreading waveform of the received spread spectrum signal. The spreadspectrum signal received at an antenna 20 is demodulated in a receiver21 to a baseband frequency by means of the carrier frequency F1generated by an RF oscillator 22. The baseband spread spectrum signal 23is fed to correlators 24 ₁. . . 24 _(N). This embodiment of theinvention comprises a separate correlator 24 for every allowed spreadingcode. For example, if the number N of allowed transfer rates is 3, thenumber N of spreading codes and correlators 24 is 3 as well. Eachcorrelator 24 ₁. . . 24 _(N) comprises a mixer 25 and a spreading codegenerator 26. The mixer 25 mixes the signal 23 with the spreading codewhereby the user signal is available at the output of that correlator 24whose spreading code corresponds with the spreading code employed by thetransmitter. At the outputs of the other correlators 24, only noise ispresent. A detector 27, for example a signal output level detector,detects which detector 24 has the user signal at its output. In thedetector 27 there is information stored on the mapping between thespreading codes and transfer rates employed by the transmitter as wellas the spreading codes of the correlator 24 ₁. . . 24 _(N). On the basisof these pieces of information, the detector 27 identifies the transferrate employed by the transmitter and provides a further processing unit28 with information on which transfer rate and the output signal ofwhich correlator 24 the further processing unit is to use. For example,if the transmitter employs a spreading code 1, the user signal ispresent at the output of the correlator 24 ₁, whereby the detector 27controls the further processing unit 28 to process the output signal ofthe correlator 24 ₁, and to use a transfer rate corresponding with thespreading code 1. As far as the invention is concerned, the functionsdetermined for the further processing unit 28 are not essential. Suchfunctions may include, for example, deinterleaving, channel decoding,error correction, etc. Generally speaking, further processing 28 may beunderstood to refer to all the receiver circuits and functions whichrequire an actual user signal or information on the transfer rate beingused. Other functions like this may, for example, include closed looppower control and multiple access interference cancellation.

The user signal is present at the output of the correct correlator 24 ₁. . . 24 _(N) after reception of only a few symbols, which means thatthe transfer rate can be detected immediately. Hence, it is possible tostart detecting the user signal after a relatively short delay, comparedto using a separate signalling channel. If necessary, it is possible toemploy buffering at the input of the further processing unit 28 tocompensate the delay required for the detection of the transfer rate.

FIGS. 3, 4 and 5 describe the second embodiment of the invention, bywhich a simpler and more economical implementation of a receiver can beachieved in a CDMA system. In the transmitter of FIG. 3, the user signalis fed from a data source 31 to a multiplier 39, in which it ismultiplied by a Rademacher waveform generated by a Rademacher generator38. The user signal modified at the multiplier 39 is applied to a mixer32 in which its spectrum is spread by a spreading code generated by aspreading code generator 35. A spread spectrum signal thus provided ismodulated in a transmitter 33 to a carrier frequency F1 generated by anRF oscillator 36, and transmitted to the radio path via an antenna 37.The transfer rate of the user signal from the data source 31 may bechanged rapidly during a call. In this example, the transfer ratesallowed for a user signal are 1R, 2R, 4R . . . R being the basictransfer rate. To every allowed transfer rate, a dedicated Rademacherwaveform is allocated for shaping the user signal in the multiplier 39.In the preferred embodiment of the invention, a Rademacher waveformhaving a frequency twice the respective transfer rate is allocated toeach transfer rate. Information on the mapping between the transferrates and the Rademacher waveforms is maintained by a control unit 34.The control unit 34 monitors the transfer rate of the data source 31,and when the transfer rate changes the unit causes the Rademachergenerator 38 to produce a Rademacher waveform allocated to the newtransfer rate. As a result, the transmitter sends a spread spectrumsignal to the radio path, the Rademacher waveform of the signalindicating the transfer rate employed.

Referring to FIG. 4, a radio frequency signal received by an antenna 40is demodulated to baseband in a reception unit 41 by an oscillatorsignal F1 from a local oscillator 42. The baseband spread spectrumsignal is applied to a mixer 44 in which the spreading code is mixedwith a spreading code generated by a generator 43, whereby the resultingoutput signal will be the user signal originally transmitted andprocessed with a Rademacher waveform. In a sampling unit 45, the outputsignal of the mixer 44 is sampled at a sampling rate which is twice thehighest allowed transfer rate (i.e. of the frequency of the Rademacherwaveform). The half symbols having the highest transfer rate areoutputted by the sampling unit 45 to a rate detector 46 and a buffermemory 47. In the buffer memory 47 the half symbols are buffered untilthe rate detection is completed. In the detector 46, the transfer rateis detected by restoring the symbols corresponding with the allowedtransfer rates. This takes place by correlating the half symbolsreceived from the sampling unit 45 in correlators 460 ₁-460 _(N). Thenumber N of correlators corresponds with the number of differenttransfer rates and Rademacher waveforms. In a correlator 1, the samplesare correlated with the first Rademacher waveform. As a result, thesymbols corresponding with the respective transfer rate are restored.The power of the symbols is integrated over a long enough time period. Asimilar procedure is carried out for each of the other Rademacherwaveforms in the respective other correlators 460. Due to the orthogonalcharacteristics of the Rademacher waveforms, only the correlator 460corresponding with the transfer rate and Rademacher code employed by thetransmitter provides an integration result deviating from zero. Theintegration result of the other correlators is zero as a response to theuser signal. In addition, the integration results contain interferenceand noise components, which are filtered to the extent required. Theoutputs of the correlators 460 ₁-460 _(N) are inputted to a control unit461 which will identify the correlator which provides the highestintegration result. Mapping between different transfer rates and theRademacher waveforms, used by the transmitter, is stored in the memoryof the control unit 461. The control unit 460 provides information onthe correct Rademacher code to the Rademacher generator 49, andinformation on the transfer rate to a further processing unit 50. Now,the half symbols buffered in the buffer memory 47 during the ratedetection are applied to a multiplier 48. In the multiplier 48 the halfsymbols are multiplied by a Rademacher waveform generated by theRademacher generator 49. As a result, the original user signal (speechor data) is fed from the multiplier 48 to the further processing unit50. The further processing unit 50 may contain similar functions as thefurther processing unit 28 of the receiver in FIG. 2.

Although FIGS. 3 and 4 show the shaping with a Rademacher code prior tothe processing with a spreading code, the processing with a Rademachercode may be carried out following the processing with a spreading code,or it may take place simultaneously, i.e. the spreading code and theRademacher code are mixed together prior to processing the actual signalwith the resulting combinatory code.

FIGS. 5A-5D illustrate different signal waveforms in the transmitter ofFIG. 3 and the receiver of FIG. 4 with a transfer rate of 2R. FIG. 5Aillustrates a data signal having a transfer rate of 2R. FIG. 5Billustrates a Rademacher waveform allocated to the transfer rate 2R, thewaveform, having the frequency 2*2R. In addition, the dotted lines inFIG. 5B illustrate Rademacher waveforms allocated to transfer rates Rand 4R. At output of the multiplier 39, a modified waveform according toFIG. 5C is obtained. This waveform is further spread, modulated andtransmitted through the radio channel to a receiver wherein thespreading is removed in a mixer 44. The output signal of the mixer 44 isshown in FIG. 5C. The half symbols at the highest data rate producedfrom the output waveform of the mixer 44 in the sampling unit 45 areapplied to the correlators in which the samples are correlated withdifferent Rademacher waveforms. The correlation results are illustratedin FIG. 5D. The correlation result of the samples and the Rademacherwaveform allocated to the transfer rate R are shown at the top of FIG.5D. The correlation depth, i.e. the symbol length, is 1/R. If thecorrelation result is integrated over this correlation depth, thecorrelation result obtained will be zero, which indicates that thetransfer rate employed is not R. The bottom part of FIG. 5D shows thecorrelation between the samples and a Rademacher waveform allocated to atransfer rate of 4R. If the correlation result is integrated over thecorrelation depth, i.e. the symbol length C_(L)=1/4R, the result isagain zero, and thus the deduction can be made that the transfer rate isnot 4R. The intermediate part of FIG. 5D shows the correlation betweenthe samples and a Rademacher waveform allocated to a transfer rate of2R. If this correlation result is integrated over the correlation depthC_(L)=1/2R, a result deviating from zero will be obtained, whichindicates that the transfer rate employed is 2R.

An advantage of the Rademacher waveforms employed in the example is acomplete orthogonality between different transfer rates, and acomputationally advantageous detection method, because the initial datafor all the correlators are common half symbols corresponding with thehighest transfer rate. Rademacher waveforms are illustrated, forexample, in the publications “Walsh Function and Their Applications”,Beauchamp, K. G., New York, N.Y., Academic Press Inc., 1975. 236p. and“Digital Communications”, Proakis, J. G., Second Edition, New York,McGraw-Hill Book Company, 1989, 186p.

However, it should be noted that although the invention is in the abovedescribed by means of Rademacher waveforms, the invention is applicablewith several different waveforms that have adequate cross correlationcharacteristics.

Although the invention is described with reference to specificembodiments, it should be understood that the description is exemplaryonly, and changes and modifications thereto are possible withoutdeparting from the scope and spirit of the invention defined in theattached claims.

What is claimed is:
 1. A method for multi-rate coding and detecting in acommunication system in which data and user information are transmittedon a radio path between a transmitting end and a receiving end, themethod comprising: allocating different signal waveforms to differenttransfer rates; selecting, at the transmitting end, a data transfer rateto be used on the radio path for transferring the user information;selecting, at the transmitting end, a signal waveform to be used on theradio path according to the selected data transfer rate; transmittingthe user information with the selected transfer rate and the selectedsignal waveform to the receiving end, said signal waveform indicatingthe selected data transfer rate; detecting, at the receiving end, thesignal waveform used on the radio path; selecting, at the receiving end,a transfer rate corresponding with the detected signal waveform; andadapting reception to the selected transfer rate.
 2. A method formulti-rate coding and detecting in a time or frequency division multipleaccess mobile communication system in which data and user informationare transmitted on a radio path between a transmitting end and areceiving end, the method comprising: allocating different time orfrequency hopping patterns to different transfer rates; selecting, atthe transmitting end, a data transfer rate to be used on the radio pathfor transferring the user information; selecting, at the transmittingend, a frequency hopping pattern to be used on the radio path accordingto the selected data transfer rate; transmitting the user informationwith the selected transfer rate and the selected frequency hoppingpattern to the receiving end, said frequency hopping pattern indicatingthe selected data transfer rate; detecting, at the receiving end, atwhich of the hopping patterns a signal is present; and selecting, at thereceiving end, a transfer rate corresponding with the detected frequencyhopping pattern for further processing of the signal.
 3. A method formulti-rate coding and detecting in a multi-carrier mobile communicationsystem in which data and user information are transmitted on a radiopath between a transmitting end and a receiving end, the methodcomprising: allocating different carriers or combinations of thedifferent carriers to different transfer rates; selecting, at thetransmitting end, a data transfer rate to be used on the radio path fortransferring the user information; selecting, at the transmitting end,the different carriers or the combinations of the different carriers tobe used on the radio path according to the selected transfer rate;transmitting the user information with the selected transfer rate and onthe selected different carriers or the combinations of the differentcarriers to the receiving end, said different carriers or thecombinations of the different carriers indicating the selected datatransfer rate; detecting, at the receiving end, at which carrier wavesor combinations of the carrier waves a signal is present; and selecting,at the receiving end, a transfer rate corresponding with the detectedcarrier waves or the combinations of the detected carrier waves forfurther processing.
 4. A method for multi-rate coding and detecting in acode division multiple access method in which data and user informationare transmitted on a radio path between a transmitting end and areceiving end, the method comprising: allocating different spreadingcodes to different transfer rates; selecting, at the transmitting end, adata transfer rate to be used on the radio path for transferring theuser information; selecting, at the transmitting end, a spreading codeto be used on the radio path according to the selected data transferrate; transmitting the user information with the selected data transferrate and on the selected spreading code to the receiving end, saidspreading code indicating the selected data transfer rate; detecting, atthe receiving end, at which of the spreading codes a signal is present;and selecting, at the receiving end, a data transfer rate correspondingwith the detected one of the spreading codes for further processing ofthe signal.
 5. A method for multi-rate coding and detecting in a codedivision multiple access method in which a user signal is transmitted ona radio path between a transmitting end and a receiving end, the methodcomprising: allocating different shaping waveforms to different transferrates; selecting, at the transmitting end, a data transfer rate to beused on the radio path for transferring the user signal; shaping theuser signal with a shaping waveform which is selected according to theselected transfer rate; spreading, at the transmitting end, the shapeduser signal with a spreading code; reproducing, at the receiving end,the shaped user signal by means of the spreading code; detecting whichof the shaping waveforms the user signal has been shaped with; andselecting a transfer rate corresponding with the detected shapingwaveform for further processing of he user signal.
 6. A method asclaimed in claim 5, wherein the shaping waveform is a Rademacherwaveform.
 7. A transmitter for multi-rate coding in a code divisionmultiple access mobile communication system, said transmitter beingarranged to send user information on a radio path with a varyingtransfer rate, said transmitter comprising: means for allocatingdifferent spreading codes to different transfer rates; and means forconveying data transfer rate information to a receiving end by using aspreading code allocated to a transfer rate on the radio path.
 8. Atransmitter for multi-rate coding in a code division multiple accessmobile communication system, comprising: means for allocating differentshaping waveforms to different transfer rates; means for conveying datatransfer rate information to a receiving end by using a signal waveformallocated to a transfer rate on a radio path; means for providing agiven spreading code; means for allocating the different shapingwaveforms and the given spreading code to the different transfer ratesin the transmitter; means for providing user information; and a signalshaper for shaping the user information with a shaping waveformallocated to the transfer rate.
 9. A receiver of a code divisionmultiple access mobile communication system, said receiver beingarranged to receive a signal containing user information transmittedfrom a transmitting end over a radio path with a varying transfer rate,said receiver comprising: means for detecting the varying transfer rate;and means for allocating different spreading codes to different transferrates, wherein said means for detecting the varying transfer ratecomprises a detector which detects a transfer rate used at thetransmitting end based on a received one of the spreading codes.
 10. Acode division multiple access mobile communication system, comprising: atransmitter; and a receiver, wherein said transmitter comprises: meansfor transmitting, on a radio path and at a selected transfer rate, aspread spectrum signal formed by mixing user information with aspreading code; means for allocating different shaping waveforms and agiven spreading code to different transfer rates; and means forconveying data transfer rate information identifying the selectedtransfer rate to the receiver by using a signal waveform allocated tothe selected transfer rate on the radio path, wherein said receivercomprises a signal shaper to shape the user information with a shapingwaveform allocated to the selected transfer rate.
 11. A system asclaimed in claim 10, wherein each of the transfer rates has allocatedthereto a Rademacher waveform which has a frequency twice as high as acorresponding transfer rate, and said receiver further comprises: amixer arranged to multiply the received spread spectrum signal by thespreading code to produce a mixer output signal; a sampling unitarranged to sample the mixer output signal at a sampling frequency whichis at least twice a highest transfer rate allowed for the userinformation; a buffer arranged to buffer said sampled mixer outputsignal for a duration of transfer rate detection; and a transfer ratedetector arranged to correlate said sample mixer output signal with eachof the Rademacher waveforms, arranged to integrate each correlationresult over an integration period which has a length of at least onesymbol at a respective transfer rate, and arranged to select a transferrate which corresponds with the Rademacher waveform having a correlationresult deviating from zero as the transfer rate of the receiver.
 12. Atransmitter for multi-rate coding in a code division multiple accessmobile communication system, said transmitter being arranged to senduser information on a radio path with a varying transfer rate, saidtransmitter comprising: a spreading code allocating mechanism toallocate different spreading codes to different transfer rates: and adata transfer rate conveying mechanism to convey data transfer rateinformation to a receiving end by using a spreading code allocated to atransfer rate on the radio path.
 13. A transmitter for multi-rate codingin a code division multiple access mobile communication system,comprising: a shaping waveform allocating mechanism to allocatedifferent shaping waveforms to different transfer rates; a data transferrate conveying mechanism to convey data transfer rate information to areceiving end by using a signal waveform allocated to a transfer rate ona radio path; a spreading code providing mechanism to provide a givenspreading code; a shaping waveform and spreading code allocatingmechanism to allocate the different shaping waveforms and the givenspreading code to the different transfer rates in the transmitter; auser information providing mechanism to provide user information; and asignal shaper to shape the user information with a shaping waveformallocated to the transfer rate.
 14. A receiver of a code divisionmultiple access mobile communication system, said receiver beingarranged to receive a signal containing user information transmittedfrom a transmitting end over a radio path with a varying transfer rate,said receiver comprising: a transfer rate detecting mechanism to detectthe varying transfer rate; and a spreading code allocating mechanism toallocate different spreading codes to different transfer rates, whereinsaid transfer rate detecting mechanism comprises a detector whichdetects a transfer rate used at the transmitting end based on a receivedone of the spreading codes.
 15. A code division multiple access mobilecommunication system, comprising: a transmitter; and a receiver, whereinsaid transmitter comprises: a spread spectrum transmitting mechanism totransmit, on a radio path and at a selected transfer rate, a spreadspectrum signal formed by mixing user information with a spreading code;a shaping waveform and spreading code allocating mechanism to allocatedifferent shaping waveforms and a given spreading code to differenttransfer rates; and a data transfer rate conveying mechanism to conveydata transfer rate information identifying the selected transfer rate tothe receiver by using a signal waveform allocated to the selectedtransfer rate on the radio path, wherein said receiver comprises asignal shaper to shape the user information with a shaping waveformallocated to the selected transfer rate.